Maturing silicon bipolar and Gallium Arsenide (GaAs) hetero-junction bipolar technologies (HBT) have made microwave monolithic integrated circuits (MMIC) feasible. A common goal for any monolithic circuit is to use design and fabrication techniques to reduce chip size, increase ease of implementation, and reduce costs in high volume production. These design and fabrication techniques should not compromise performance, and if possible, should improve performance.
Three conventional variable gain amplifiers exist which use bipolar technology. A first approach uses a current steering method in which current is supplied across an amplifier load resistor to change a voltage swing through the load resistor. The first current steering approach is implemented using emitter-coupled logic (ECL) and is compact in size. However, the first approach is noisy due to additional transistors needed to accomplish current steering. The current steering approach is somewhat stable at microwave frequencies but is not desirable at high frequencies. Third order intermodulation performance (IP3) is generally limited.
A second approach adjusts current through an amplifying transistor to change an effective transconductance. The second approach has a favorable or low noise factor/gain ratio and an unfavorable or high IP3 gain ratio, and requires a biasing scheme for controlling gain.
A third approach uses electronically controllable resistors in parallel or series feedback amplifiers. While the third approach has acceptable gain, bandwidth, linearity, and noise, an expensive off-chip discrete variable resistance is required.
Variable gain amplifiers according to the third approach typically use a p-i-n diode as a variable resistance element in the parallel or the series feedback path. Two discrete components are required for the third approach: a transconductance device, for example a FET or bipolar device; and a p-i-n diode. The two discrete components are typically integrated into an expensive multi-component hybrid. Furthermore, VGA designs using the p-i-n diodes typically include area-consuming spiral inductors in the feedback path and matching networks to obtain high frequency (e.g. microwave) response.
Fabrication of the spiral inductor on a monolithic chip requires careful modeling, a complicated fabrication process, and large surface areas on the Silicon or GaAs wafer. The surface area required for an inductor increases as the inductance value increases. Since high frequency applications typically require inductors with high inductance, the surface area of these inductors adversely affect chip size.